This relates to construction machines, such as earth grading, bulldozing, and paving machines, in which it is desired to control the position of a blade or other machine element carried by the machine, or to control the direction of machine movement or some other machine function. More specifically, this relates to such machines in which the control is to be effected in regard to a reference surface or point of interest on a reference surface.
In conventional grading vehicles, an operator of the vehicle will set the height of the blade on a surface to be graded at a particular level relative to a reference surface. The reference surface may be an adjacent portion of ground, a standard string line running parallel to the direction of machine movement against which vertical measurements are to be taken, or a roadside curb. In the contact method of following, the string line or other reference surface is actually contacted by a mechanical follower that slides over the reference surface to detect changes in its elevation. For example, a light wire element may be used to track along the top of a string line, whereas a ski-like follower may be used to ride over adjacent curbs or ground areas. The vertical movement of the wire element or the ski-like follower is monitored through an electromechanical linkage, providing an input for the machine control system. Mechanical contact follower systems can encounter rough use from continued movement over various surfaces, making dependability an issue.
Acoustic systems have been used to follow the reference surface without the need for physical contact between the surface and a follower. In some systems, such as shown in the U.S. Pat. No. 4,733,355, to Davidson, an acoustic sensor uses an acoustic signal echo to measure distance, and uses timing windows to determine the approximate round trip time of the return echo. The system is typically used over a period of many hours, with the result that the ambient temperature in the vicinity of the sensor changes, changing the density of the air and the speed of sound. This, in turn, causes the sensed distances to change, since the sensor results depend on travel time of the acoustic pulse. Wind and transient thermal currents can also degrade the accuracy of such a system. Furthermore acoustic systems may have a relative small area over which they can sense the presence of the surface. Finally, the such acoustic systems are only capable of determining the height of the point that is closest to the sensor, i.e., its Z coordinate, and are not able to determine its X and Y coordinates.